On more than one set of cast aluminum covers I had to mill the sealing surface. I don't see this as an issue in your case, but in one case I had to take 0.080" off the boss where the bolt goes through because it was hitting the head there and would never properly compress the sealing surface. I found this by putting a piece of paper between the gasket and head and it would slip right out.

I have had the best luck milling the valve cover sealing surfaces flat and gluing a rubber gasket to the cover. Degrease the head sealing surface and install. Usually have zero leaks with this approach.